The Fermi energy of graphene can be tuned dynamically by the
applied voltage, which is the key advantage of graphene plasmonics
compared to noble metal based plasmonics. Therefore, we
study how the variation of Fermi energy impacts the absorption
spectrum. As shown in Fig. 3(a), as the Fermi energy increases from
0.2 eV to 0.8 eV, the resonant wavelength decrease from 15 lm to
7.5 lm, indicating the spectrums can be dynamically tuned at a
very wide range by changing the Fermi energy of graphene. The
absorption peaks is no longer 100% at other Fermi energies except
at 0.4 eV. This is because the resonant wavelength is changed while
the substrate thickness is fixed, thus the condition n h = m ka/4 can
no longer be fulfilled, resulting in the variation of the coupling rate
c1. Despite of this, the absorption rate is still higher than 60% at
such a wide wavelength range, which could be useful in many
practical applications.